amps for new a/c?

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Old 06-04-09, 10:20 AM
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amps for new a/c?

I am replacing an old 220v a/c with a new one. The old feed is (2) 20amp breakers. The new unit requires 30amps. Do I need to change out the old breakers to 30amps or can I feed the new one with the esisting 40amps? - thanks!
 
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Old 06-04-09, 10:54 AM
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You have a double-pole 20A breaker (not to be confused with 40A).

If your new unit requires 30A @ 240v (which most common condensers do), you'll need to run new 10/2 wire and connect it to a new double-pole 30A breaker. Your current wire is most likely 12ga, and will not be usable with a 30A breaker.

I'm assuming that you are installing a condenser, and not a window AC or some other component that needs 30A at 240/120v. In that case you'd need to use 12/3 (hot-hot-neutral).

Don't forget you'll need a disconnect outside next to the condenser, a flexible whip to connect the disconnect to the AC (using THWN, not Romex/NM-B), and you'll need a standard convenience receptacle within 20'.

Of course, post back if you have any other questions!
 
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Old 06-04-09, 10:56 AM
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Originally Posted by marstine View Post
I am replacing an old 220v a/c with a new one. The old feed is (2) 20amp breakers.
That's one 20A double-pole breaker, not 40A.

The new unit requires 30amps. Do I need to change out the old breakers to 30amps
The new A/C requires a 30A double-pole breaker and new #10 AWG copper wiring. The old unit only uses #12 wiring which is not heavy enough to support the new unit.
 
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Old 06-04-09, 01:26 PM
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Check the nameplate on the new unit and look for the minimum circuit ampacity. Wire sizing for AC units is slightly different than regular circuits. Your #12 may be ok for the 30 amp unit.
 
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Old 06-04-09, 06:24 PM
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I am not sure if you are talking about a window unit or pad mounted condensing unit. Either way you will need to beef up your wire size. #10 guage wire is rated for 30A. If your old unit was being fed with #12 then you will need to increase the size to #10. You will also need to protect the new wire with a two pole 30A breaker. If this is a pad mounted condensing unit you will also need to install a new 30A disconect as close to the new unit as possible. You can buy what they call a pull out disconnect for about twenty dollars. This can be screwed right to the wall next to the new unit and connected to the unit via liquid tight flexible conduit. I hope this helps.
 
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Old 06-06-09, 08:56 PM
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The rules for Air Conditioning is different then what most folks think as "normal", it is not unheard of to see 12 gauge wire on a 40 ampere fuse or circuit breaker & still be code compliant, that issue confuses a lot of professional electricians & dingbat home inspectors. See NEC article 440.

The reason that is allowed is because the compressor has internal overload protection so the fuse or circuit breaker is only providing short circuit protection, the A/C is wired for the minimum & fused to the maximum allowed. there is a label on newer A/C's stating the minimum* circuit amperes & the maximum size fuse or circuit breaker & any installation will need to comply with that labeling and NEC code.

*One could just wire & fuse to minimum but as compressor ages it will be harder to start & then will be likely to trip the breaker.
 
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Old 06-06-09, 09:10 PM
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Originally Posted by Norcal View Post
The rules for Air Conditioning is different then what most folks think as "normal", it is not unheard of to see 12 gauge wire on a 40 ampere fuse or circuit breaker & still be code compliant, that issue confuses a lot of professional electricians & dingbat home inspectors. See NEC article 440.

The reason that is allowed is because the compressor has internal overload protection so the fuse or circuit breaker is only providing short circuit protection, the A/C is wired for the minimum & fused to the maximum allowed. there is a label on newer A/C's stating the minimum* circuit amperes & the maximum size fuse or circuit breaker & any installation will need to comply with that labeling and NEC code.

*One could just wire & fuse to minimum but as compressor ages it will be harder to start & then will be likely to trip the breaker.
Norcal have very good point related to the Air Condtioning system with sealed compessor motor and also let me add one more item some of the standard electric motor do have interal overload protections they can use NEC Art 430 as well.

I know some of readers will get confused in here but the code is very clear on this matter and it been written for last 20+ years but becarefull if you use Art 430 that is good for hardwired motors but for cord et plug attached motors the rules do not apply to this.

I know it kinda compaited but once you understand the rules it is not too bad.

Merci,Marc
 
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Old 06-06-09, 09:14 PM
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Thank you for adding that "Window Shakers" do not get to use those rules , a window type unit will be marked by the manufacturer as to the circuit they are allowed to be plugged into.
 
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